This invention relates generally to microlens arrays for various applications including increasing the fill factor of photosensitive arrays.
Conventional imaging systems may include a light sensing element such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) sensor. The sensor may include one or more metal layers and interconnects, interlayer dielectric (ILD), a passivation layer such as a nitride layer, a color filter array (CFA), planarization over the CFA and a microlens array over the CFA. Conventionally, microlens arrays are formed using positive photoresist materials.
Conventional microlens fabrication involves forming a nitride passivation, and then opening a bond pad. A polyimide layer is used to transfer a pattern to the positive photoresist. The polyimide layer is removed and a color filter array is formed. Thereafter, the device is planarized. Microlenses are defined in positive photoresist and then an ultraviolet (UV) bleaching step is used to improve the transmissivity or optical clarity of the positive photoresist.
Conventional processes may produce striations. In conventional processes, the bond pad opening is formed before CFA formation. The topographic variation due to the surface cavity in the bond pad areas is a direct source of streaking patterns in CFAs which may later cause streaking in the resulting images.
Photobleaching may be used with positive photoresist because of the yellowing that may occur during processing. In addition positive photoresists are inherently not transparent, even after a hard bake. This is believed to be due at least in part to the presence of photoacid compounds in positive photoresist. Thus, the positive photoresist based microlenses are photobleached using a deep ultraviolet (UV) source. Even with bleaching, a yellowing problem may arise upon exposure to heat and humidity.
With existing positive photoresist microlens formation processes, the bond pads may be left with residues because the bond pad opening is formed before the microlens is formed. Thus, a final bond pad area opening may be covered with residuals preventing good contact to the bond pad.
The thermal stability of positive photoresists is also limited. When exposed to high temperatures, positive photoresist may change shape or lose its optical clarity. Thus, it is generally desirable to avoid high temperatures with positive photoresist based microlens arrays. However, avoiding high temperatures prevents using the surface mount process where the silicon chip and microlens can be heated up to over 200 degrees during the solder reflow.
Additionally, in conventional processes, the regions adjacent to the microlenses, which are not situated over photosensitive elements, are subject to scratching during packaging because the nitride passivation is completely exposed. This scratching may ruin the devices and be an undetected source for light contamination.
Thus, there is a continuing need for improved photosensitive devices having improved microlens arrays.
In accordance with one embodiment, a microlens may include a light collecting element. The element may be formed of negative photoresist.